Novel device technologies, such as those using third generation semiconductor materials (e.g., GaN) to construct high electron mobility transistors (HEMT) dramatically improve both switching and conduction efficiency, due to the wide bandgap property of these new materials and corresponding two-dimensional electron gas (2DEG) from the heterostructure formed by AlGaN and GaN layers. These devices are typically lateral devices, which means that current is flowing close to the surface.
Many power electronics applications typically require semiconductor devices capable of carrying current in the range of 1 A to 100 A. In lateral power devices such as GaN HEMT, current rating depends on the width of the current flowing path (from drain to source or vice versa) that translates into a very narrow strip structure, which is impractical according to today's semiconductor manufacturing and assembly standard. Interdigitated electrodes have been designed to improve the current carrying capability.
Interdigitated electrodes for semiconductor electronic and optoelectronic devices have the advantage of efficient use of wafer area and low overall contact resistance. A commonly used set of interdigitated electrodes consists of rectangular intervening fingers connected to two electrode bases, such as drain 11 and source 12 in FIG. 1. All metal structures are rectangle and are easy to implement in layout and fabrication.
Interdigitated electrodes have been in common use for the cathode and anode of photodiodes (PD) and sensors [1][2][3], and for the source and drain of field effect transistors (FETs) [4][5] with the gate located between.
Layout using interdigitated electrodes is also related to packaging since the electrical wiring must be properly bonded to make contact with the bases of the electrodes. To reduce resistance, the distribution of current flow path from the wiring to the fingers must be taken into account.
For FETs, previous work [4] proposed using a rounded (semi-circle) shape for the fingertips of a rectangle [4] to reduce corner effects. The geometry of the fingers was also converted from rectangular to trapezoidal to reduce current crowding (thus heating) [5].
For PDs and sensors, previous efforts to optimize the geometry includes the following: 1) semicircular-shaped interdigitated electrodes were used to reduce capacitance and thus increase speed [1]; 2) star-shaped electrode base with curved fingers was used, for the purpose of reducing the electrode shadowing effect in order to increase photo detection sensitivity [2]; 3) symmetric cycles and polygons were used for the fingers in order to fit to the circular acoustic device of interest and for ease of processing [3].
Previous work [5] described the use of a trapezoidal finger shape which resulted in improved current flow distribution and reduced resistance. However, there are limitations related to high fields at corners and inability to spread the high fields to other parts of the fingers.
In addition, existing layout designs potentially have high device on-state resistance and/or low break down voltage due to non-uniformity in current flow or field distribution.